the primary difference between the two crystals you're describing /u/trap2110 is the time and method used to achieve these isolates. a longer, slower recrystallization will yield larger, more visibly pure crystals, while a faster or more shock-based crystallization will yield smaller crystals or dust. This concept then determines potency/THCa concentration, because a slower-forming large crystal will have significantly less impurities, due to the lack of physical room for impurities to find a home between the closely packed THCa molecules that have crystallized into a lattice formation. a smaller crystalline structure or powder would mean there is more room for the presence of "impurities", whether those impurities are in the form of terpenes, other cannabinoids, or potentially chlorophyll, other pigments, or plant fats.

Edit: to answer your original question, this falls under the concept of "boiling point elevation", where the presence of "impurities" of any kind will increase the boiling point of the compound in question as a function of the mass of the impurity. TL:DR the larger crystals of THCa will have a more accurate boiling point and will need to be taken at a little bit lower temperatures.

Grinding it to a fine consistency can cause the butane to tunnel and not allow all of the plant material to come into contact with the solvent. I'd recommend breaking the nugs up to about the size (diameter) of a penny or nickel. Then the butane can come into contact with all of the plant matter and also not preferentially dissolve as many accessory pigments and as much chlorophyll.